This invention relates generally to a method and apparatus for image correlation and, more particularly, to a ferroelectric optical image correlation (FOIC) device.
The property of ferroelectric ceramics such as lead lanthanum zirconate titanate (PLZT) to store information has been known for many years. This relates to the property of ferroelectric ceramic materials to become remanently polarized when an electric signal is applied to the material. The combination of a positive bias voltage and light exposure causes the ferroelectric domains to reorient or reverse in the exposed regions while remaining unchanged in the unexposed regions. In studies of the maximum resolution of images stored in PLZT bulk ceramic plates, it was found that the minimum size of a pixel (image resolution element) is approximately four to five grain diameters. The minimum grain size that has been obtained in PLZT bulk ceramics is about 1 .mu.m and the minimum pixel size is about 4 to 5 .mu.m. This process of image storage using PLZT polycrystalline plates has been found for these reasons to be unsuitable for use in an FOIC because of the poor resolution.
Additionally, when light is transmitted through a PLZT polycrystalline ceramic plate, it encounters refractive index mismatches at grain and domain boundaries which produce depolarization and light scattering. For FOIC applications, light scattering in the storage medium greatly reduces the resolution of stored images and destroys the image correlation capabilities of the FOIC. In this regard, the switching time determines the lower boundary with respect to the image storage time. Switching times on the order of tens of microseconds are typical for PLZT ceramic plates. In many applications of FOIC devices, such switching times are inadequate.
Because of the very large information content in an optical image, it is generally difficult to process such an image on a real-time basis using even the fastest digital computing techniques. Consequently, there is a need for various analog devices which, because of their parallel nature, can process optical images on a real-time basis.
Correlation devices have a wide variety of present applications and an even wider variety of future uses. These devices provide a way for determining whether an externally perceived object matches or looks the same as a known object. A few examples of systems which either use or could use correlation devices include intrusion detection systems which signal when the scene changes, terrain identification systems which signal when an image is matched with a map, counterfeit detection systems which signal when a document examined does not match the pattern of an authentic document, and critical alignment systems which signal when a target image such as a component of a machine is properly positioned.
In all of the above examples of correlation devices, one of the most important requirements is that the device be able to perform the necessary correlations at great speed without the need for many optical components and the use of slow mechanical scanning components. These latter components are simply too bulky and are unsuitable in many applications where the target image is in motion.